I don't think people are actually surprised she changed her mind whatsoever. The problem with Cecilia's 180° is that it is way too brutal and didn't have any transition or gradual interactions changes. Plain and simply, it is as if she became a totally different character after a flag and keep go back and forth.
It is just like flipping a switch, and that's from your cookie cutter Oujo archtype, which makes the deal harder to accept than a tsundere.

Yes, I would have a loved a more casual transition as well. A good example would be Felli in Regios. It took a few episodes and the transition was more gradual. As mentioned by some of the novel readers, it seems that Cecilia's background story was dropped, which probably would have helped explain her sudden change of heart.

Well, I am still enjoying this series, if lacking in explaination at times.